The performance of a mass spectrometer can be reduced by contamination of its components, such as ion sources. During operation of a MALDI desorption ion source, for example, a sometimes visible coating of organic material can build up on the electrodes. In the prior art, such coatings on ion-optical devices in mass spectrometers are described by Girard et al. in the Journal of Chromatography Science, 2010 October, 48 (9), 778-779, and in the article by Kenneth L. Busch entitled “Ion Burn and the Dirt of Mass Spectrometry”, online publication, Sep. 1, 2010. The insulating organic coating becomes charged when the ion source is in operation and thus generates an electrical interference field which is superimposed onto the desired electric field between the electrodes and the MALDI sample support when the desorption ion source is in operation. This interferes with the acceleration process. Field changes, in particular, interfere with the focusing properties of the accelerating electrodes. Consequently, the ion beam is no longer focused accurately onto the detector.
A noticeable effect of such a coating can be a decrease in ion throughput to the mass analyzer connected to the ion source. The reduced ion throughput in turn requires the additional acquisition and summation of spectra in order to maintain a specific quality level for the mass spectra. The reduction in the ion throughput also limits the number of analyses which are possible per sample, and reduces the detection limit of the mass spectrometer.
Girard et al. describe a method whereby a simple reversal of the polarity of the ion source, which changes the polarity of the ions to be analyzed, can neutralize the charging effect. Since a MALDI method generates ions of both polarities, the polarity of the accelerating field would therefore have to be reversed for the analogous application of the method according to Girard et al. However, this method only addresses the symptoms of the loss of throughput in the ion source, and promises only a short-term effect.
Irrespective of the short-term solution mentioned above, there is therefore regularly a need to remove the coating and thus restore the performance of the mass spectrometer. Sometimes, if the cleaning is not able to restore something approaching the ideal state of an ion-optical device, it must be replaced by a new, clean one. Ion-optical devices can be taken to include all of the elements of a mass spectrometer and/or of an ion source on which deposits can form, for example, accelerating and/or ground electrodes of an ion source, and also injection capillaries, multipole rod systems, ion funnels comprising ring electrodes, ion deflectors (condensers) and similar.
Cleaning methods are known in the prior art with which the contamination can be at least partially removed. U.S. patent application U.S. 2004/0163673 A1 (Holle et al.) describes a sample support dummy with bristles, for example, which can remove interfering coatings by “scrubbing”, and a spray cleaning device which utilizes the low pressure in the vacuum chamber of the ion source in order to direct a jet of solvent onto the accelerating electrodes and to dissolve and remove deposits by its impact. German patent application DE 10 2008 008 634 A1 (Holle et al.) discloses a method where the coating is removed by local heating.
A further method for removing the coating, which is still used in practice, is to clean the electrodes manually after venting and opening the mass spectrometer. The cleaning is usually carried out with solvents such as ethanol or acetone, but when the contaminations are stubborn the electrodes can also be abraded with cleaners containing abrasive agents (including toothpaste, for example). Since the confined space makes it difficult to clean the ion-optical assembly while it is inside the mass spectrometer, and the aim is to avoid contaminating neighboring components with the dirt removed during the cleaning process, the ion-optical assembly is usually uninstalled. If the mass spectrometer is vented during the disassembly, it often takes some hours until the necessary operating vacuum is restored after the ion-optical assembly has been cleaned and re-inserted.
The removal and cleaning themselves are usually steps which can be carried out in a straightforward manner They require a certain degree of experience, but no special knowledge. These steps can thus also be carried out without any major difficulty by members of staff who have been given appropriate instruction. It becomes difficult, however, when the removed parts have to be reinserted in their position in the mass spectrometer. The necessary use of electromagnetic forces and fields to control and manipulate ions means that the ion-optical devices have to be positioned within narrow tolerances. These tolerances should be no larger than a few tens of micrometers. For example, the separation, perpendicular to the surface, between a MALDI sample support with sample applied to it and the first accelerating electrode essentially determines the acceleration path of ions on their way to the mass analyzer, and thus the kinetic energy they accumulate along the path. The control and correct adjustment of this kinetic energy is decisive for the operation of a time-of-flight mass spectrometer. Deviations of the separation, perpendicular to the surface, between the sample and the accelerating electrode (also indirectly via a lateral offset/shift) can therefore have a significant negative influence on a mass spectrometric analysis.
In addition, ion-optical devices have to be supplied with voltages. This means that electrical supply lines, which at present usually have screw connections, must be detached during removal of the ion-optical assembly and reconnected during reinstallation, which requires additional manual measures.
For this reason, specially trained staff from the manufacturer or its appointed dealers are often required to reinstall an ion-optical assembly which has been removed for cleaning. This may involve realigning the reinstalled ion-optical assembly in the mass spectrometer in order to ensure that it is reinserted with high positioning accuracy. If there are no alignment marks or similar in the mass spectrometer, it is often not just that the ion-optical assembly must be realigned with respect to the mass spectrometer. It may also be necessary to realign other components of the mass spectrometer, such as a reflector or a detector (in two planes), not to mention additional fine tuning of the supply voltages. The man-hours required for such maintenance work are considerable, and also costly for the user of the mass spectrometer, who must pay the travel expenses of the specialist personnel, for example.
U.S. patent application U.S. 2009/0242747 A1 (Guckenberger et al.) discloses a mass spectrometer in which an ion source and various ion-optical elements are integrated in a sub-unit. The sub-unit is removed from the mass spectrometer in order to clean away the contamination that builds up during operation, and reinserted, whilst maintaining the vacuum.
U.S. Pat. No. 7,601,951 B1 (Whitehouse et al.) describes an atmospheric pressure ion source which is designed so that all or some of the vacuum components, such as ion-focusing and ion-transporting electrostatic lenses and ion guides and two or more vacuum stages integrated into a unit are removed from an ion source or vacuum housing
U.S. Pat. No. 7,667,193 B2 (Finlay) discloses a mass spectrometer with modular design in order to provide a user with an appropriately personalized analytical device by inserting a personalized analytical module. High positioning accuracy when reinserting the modular components is assumed in all three documents, largely without providing design details
U.S. Pat. No. 6,797,948 B1 (Wang) and U.S. Pat. No. 4,745,277 A (Banar et al.) disclose assembly plans for a multipole rod set and a heated filament ion source in a mass spectrometer, respectively, in more detail.
At the beginning, MALDI ion sources were referred to specifically. The invention to be presented below should not, however, be limited to special types of generation or guidance of ions in a mass spectrometer. Similar considerations can also be made for electrospray ion sources, electron impact ion sources, ion sources with chemical ionization, and others.
There is a need to provide a device for use in a mass spectrometer which allows an ion-optical device or assembly to be removed, cleaned and reinserted with high positioning accuracy without the need for special knowledge. In particular, the device should obviate the need for complex adjustments requiring special knowledge after the reinsertion.